1. Technical Field
The present invention relates to an image forming apparatus and image forming method applicable to a tandem system color printer, color photocopier or multi-functional peripheral which is provided with a photosensitive drum for providing collective exposure of an electrostatic latent image, by a unit of line, from a LED print head unit wherein light sources are arranged linearly for each image forming color, whereby a colored image is formed by superimposition of colors on an intermediate transfer belt.
2. Description of Related Art
A tandem system color printer, a color photocopier and the related color multi-functional peripheral have come into widespread use in recent years. In this type of image forming apparatus, when the red (R), green (G) and blue (B) of a colored image are to be reproduced, an LPH (Line Photo diode Head) unit equipped with laser light sources arranged linearly to provide collective exposure for each line is provided for each image forming color, and the toner images of yellow (Y), magenta (M), cyan (C) and black (BK) are formed by the photosensitive drums for respective image forming colors, whereby toner images of various colors formed by photosensitive drums for colors are superimposed on an intermediate transfer belt. Color toner images superimposed on the intermediate transfer belt are transferred to a desired sheet of paper and are ejected after being fixed.
In the tandem system color image forming apparatus, if there is a fluctuation (irregularity) in the rotational velocity of a photosensitive drum, the printed image may become irregular, and color misregistration and line misregistration may occur to the colored image formed by superimposition of single colors by respective image forming units in some cases.
In connection with the aforementioned tandem system color printer, JP2000-089640A (Japanese Unexamined Patent Application Publication No. 2000089640 (FIG. 1 on page 3)) discloses an image forming apparatus. This image forming apparatus includes a rotating operation detecting device, signal filter and write timing control device. When correcting the rotational irregularity of the photosensitive drum, the rotational irregularity of the photosensitive drum is detected by the rotating operation detecting device and the rotational irregularity detection signal is outputted to a signal filter. The signal filter picks up the low frequency component signals after repetitive components have been removed from the uneven rotation detecting signal, and the low frequency component signals are outputted to a write timing control device. The aforementioned low frequency component signals have been produced by drum eccentricity. The write timing control device calculates the rotational fluctuation amount from the low frequency component signals, and the image write timing of a write unit is determined based on this rotational fluctuation amount. The image forming apparatus constituted in this manner ensures accurate and speedy correction of the rotational irregularity of the photosensitive drum.
In the sheet conveyance control apparatus of the image forming apparatus shown in JP2005-284019A (Japanese Unexamined Patent Application Publication No. 2005-284019 (FIG. 2 on pages 6 to 7)), a loop sensor flag rocked by being pushed by the conveyed sheets, and a plurality of loop sensors (photo interrupters) for sensing the flag being rocked are provided around a fixing inlet conveyance guide. The magnitude of the loop of the conveyed sheets are detected in gradations at least two positions. This structure allows tough sheets to be conveyed with the loop amount kept smaller, and permits less tough sheets to be conveyed with the loop amount kept greater. This arrangement minimizes pulling or pushing of sheets by a conveyance belt of static suction and a fixing apparatus, and provides high-quality printing with the minimized color misregistration.
JP1995225544A (Japanese Unexamined Patent Application Publication No. Hei. 7-225544 (FIG. 1 on page 6)) discloses an image forming apparatus as the aforementioned tandem system color printer. In this image forming apparatus, a photosensitive drum is provided for each image forming color and a plurality of photosensitive drums are belt-driven by one drive source. The shaft of each photosensitive drum is equipped with an encoder (velocity detecting device). The fluctuation in the rotational movement anticipated from the rotational velocity information obtained from each axis is stored in advance and the record timing is controlled based on this rotational movement. The aforementioned structure of the image forming apparatus eliminates the possibility of color misregistration when the colors are superimposed on the intermediate transfer member.
The aforementioned image forming apparatus of the conventional art has a problem in that color misregistration or line misregistration has occurred to the image formed on the transfer paper due to the fluctuation in the rotational velocity of the photosensitive drum. One of the solution to this problem is to adopt the rotation detecting system of JP2000-089640A, wherein a phase detecting sensor is used for sequential phase adjustment between the color photosensitive drum shaft and monochromatic photosensitive drum shaft.
In actual practice, however, the rotational velocity of the photosensitive drum is also affected by the load fluctuation resulting from passage of sheets. For example, for thick or large-sized sheets with greater frictional force, the load is increased and the rotational velocity of the photosensitive drum is reduced. Conversely, for thin or small-sized sheets with smaller frictional force, the load is decreased and the rotational velocity of the photosensitive drum is increased.
One of the ways to correct the adverse effect of the passage of sheets is to use the image forming apparatus disclosed in JP2005-284019A. In this image forming apparatus, the sheets are classified into four stages, for example, according to the size of the loop and adjustment is made in four stages. This makes it difficult to achieve accurate correction in response to the fluctuation in the rotational velocity of the photosensitive drum.
In these examples of conventional art, a correction value is obtained while an image is actually formed on paper. When images are formed on a plurality of transfer members on a continuous basis, there may be a delay of at least one sheet in the feedback of the correction value. Alternatively, this requires measurement to get the correction value in conformity to the type of sheets having been modified, with the result that the image formation time as a whole is prolonged.
The object of the present invention is to solve the aforementioned problems and to provide an image forming apparatus and image forming method wherein a high-quality image can be formed on a plurality of types of transfer members having different size, material, frictional force and others, and continuous formation of high-quality images is provided without the time being prolonged, when images have been formed on a plurality of types of transfer members have been on a continuous basis.
In the tandem system color image forming apparatus, the rotating operation of the photosensitive drum and exposure operation of the LPH unit are asynchronously controlled. When the irregular fluctuation in the drum rotational velocity is removed, the rotational velocity fluctuation distribution of the photosensitive drum is measured, and reference is made to the amount of correction that offsets this rotational velocity fluctuation distribution, whereby the image write reference signal (reference index signal) is corrected.
However, even if the reference index signal is corrected at the cycle of canceling the irregularity in rotational velocity fluctuation, there is concern about the possibility that correction is disabled when the time period of one rotation of the drum is prolonged due to the processing (image formation) conditions such as the type of sheets.
Incidentally, in the image forming apparatus disclosed in JP2000-089640A and JP2005-284019A, the amount of correction that cancels the irregularity in rotational velocity fluctuation of the photosensitive drum is calculated prior to formation of an image so that a correction data table is created. When an image is formed, the amount of correction in this correction data table is referenced, and the image write reference signal is corrected. If this method is adopted, there may be a shortage in correction data in some case.
In this case, one round cycle of drum is divided into “n” parts and the velocity fluctuation distribution is measured. Although “n” correction values corresponding to “n” blocks have been created on the correction data table, the rotational velocity of the photosensitive drum is reduced by a change in the image forming conditions such as the type of sheets at the time of forming an image. Thus, even if all the “n” pieces of correction data are referenced in time period of one rotation of the drum, referencing in one round cycle of drum does not terminate, and an attempt is made to reference the (n+1)th correction value which is actually non-existent. Such a problem may arise in this case.
Patent Documents JP2000-089640A and JP1995-225544A use the method of correlating the exposure interval with the drum rotational velocity. There is no solution to the problem of an operation error wherein there is a shortage of correction data. Consideration is not given to an error system wherein the number of factors in the correction data for referencing is exceeded. Thus, JP2000-089640A and JP1995-225544A fail to keep the irregularity of an image within the range of the irregularity in rotational velocity fluctuation.
The present invention solves the aforementioned problems. Thus, the object of the present invention is to provide an image forming apparatus and image forming method capable of keeping the irregularity of an image within the range of the irregularity in rotational velocity fluctuation, without any correction failure even if one round cycle of drum has been prolonged by image forming conditions.